Brain Crease material
There are three options.



You could leverage the "Pointiness" value. If you use it as a mix factor, you'll be able to blend between different materials based on the geometry. The benefit of this is you don't have to bake anything.

More in the Manual.





You could also create "Dirty Vertex Colors" in vertex paint mode and use that as the mix factor. It has the benefit of not having to bake anything, but its quality is directly related to how much geometry you have (more geometry will look better). BlenderGuru has a great tutorial about how to add dirt with this tool.






The somewhat more standard answer (and probably more appropriate) is to use an Ambient Occlusion map as the mix factor. You have to bake the AO to a texture of some kind, but it has the benefit of being very accurate and providing somewhat more control than the other options.

More in the Manual.





Technically a fourth option is to combine one or all of those techniques.